The high degree of resistance which most cancer cells display to treatment with antifolate compounds, such as methotrexate, presents the need for the production of new chemotherapeutic agents. This research project is aimed at determining the molecular basis of specificity of pteridines and pteridine-like compounds so that in the development of new folate antagonists toxic side effects can be minimized. We intend to study the structure-action relationship of differently substituted pteridines and pteridine-like compounds as cofactors and inhibitors of phenylalanine hydroxylase, tyrosine hydroxylase, and tryptophan hydroxylase, each of which use a pteridine, tetrahydrobiopterin, as cofactor. The effects of the compounds will also be determined on: (1) dihydropteridine reductase, the enzyme which in vivo regenerates tetrahydrobiopterin during the reaction of the aromatic amino acid hydroxylases; and (2) dihydrofolate reductase, which in addition to converting the dietary form of folic acid to the active cofactors form, tetrahydrofolic acid, also converts the dietary form of biopterin to the cofactor form, tetrahydrobiopterin. The appropriately substituted pteridines, if not available, will be synthesized, so that the molecular requirements for binding and for cofactor or substrate activity can be determined with each of the above enzymes. The results of this project will yield information necessary for the development of folate antagonists which selectively inhibit dihydrofolate reductase, but which are effective against a broader range of cancer than is methotrexate.